Until now, our Serializable mode has in fact been what's called Snapshot
Isolation, which allows some anomalies that could not occur in any
serialized ordering of the transactions. This patch fixes that using a
method called Serializable Snapshot Isolation, based on research papers by
Michael J. Cahill (see README-SSI for full references). In Serializable
Snapshot Isolation, transactions run like they do in Snapshot Isolation,
but a predicate lock manager observes the reads and writes performed and
aborts transactions if it detects that an anomaly might occur. This method
produces some false positives, ie. it sometimes aborts transactions even
though there is no anomaly.
To track reads we implement predicate locking, see storage/lmgr/predicate.c.
Whenever a tuple is read, a predicate lock is acquired on the tuple. Shared
memory is finite, so when a transaction takes many tuple-level locks on a
page, the locks are promoted to a single page-level lock, and further to a
single relation level lock if necessary. To lock key values with no matching
tuple, a sequential scan always takes a relation-level lock, and an index
scan acquires a page-level lock that covers the search key, whether or not
there are any matching keys at the moment.
A predicate lock doesn't conflict with any regular locks or with another
predicate locks in the normal sense. They're only used by the predicate lock
manager to detect the danger of anomalies. Only serializable transactions
participate in predicate locking, so there should be no extra overhead for
for other transactions.
Predicate locks can't be released at commit, but must be remembered until
all the transactions that overlapped with it have completed. That means that
we need to remember an unbounded amount of predicate locks, so we apply a
lossy but conservative method of tracking locks for committed transactions.
If we run short of shared memory, we overflow to a new "pg_serial" SLRU
pool.
We don't currently allow Serializable transactions in Hot Standby mode.
That would be hard, because even read-only transactions can cause anomalies
that wouldn't otherwise occur.
Serializable isolation mode now means the new fully serializable level.
Repeatable Read gives you the old Snapshot Isolation level that we have
always had.
Kevin Grittner and Dan Ports, reviewed by Jeff Davis, Heikki Linnakangas and
Anssi Kääriäinen
String are converted to UTF8 on the way into perl and to the
database encoding on the way back. This avoids a number of
observed anomalies, and ensures Perl a consistent view of the
world.
Some minor code cleanups are also accomplished.
Alex Hunsaker, reviewed by Andy Colson.
If the foreign table's rowtype is being used as the type of a column in
another table, we can't just up and change its data type. This was
already checked for composite types and ordinary tables, but we
previously failed to enforce it for foreign tables.
Make sure it's clear that the prohibition on adding a column with a default
when the rowtype is used elsewhere is intentional, and be a bit more
explicit about the other cases where we perform this check.
Remove the claim that ALTER TABLE .. SET DATA TYPE is the fastest way of
rewriting a table, since it no longer is.
Noah Misch and Robert Haas, based on a suggestion from Tom Lane.
This fixes make distprep, and seems more robust in other ways as well.
Some special handling is required because errcodes.txt is needed by
some stuff in src/port, but just by src/backend as is the case for the
other generated headers.
While I'm at it, fix a few other things that were overlooked in the
original patch.
src/pl/plpgsql/src/plerrcodes.h, src/include/utils/errcodes.h, and a
big chunk of errcodes.sgml are now automatically generated from a single
file, src/backend/utils/errcodes.txt.
Jan Urbański, reviewed by Tom Lane.
Add the current xlog insert location to the response of
IDENTIFY_SYSTEM, and adds result sets containing start
and stop location of backups to BASE_BACKUP responses.
Prior to 9.0, restartpoints never created, deleted, or recycled WAL
files, but now they can. This code makes log_checkpoints treat
checkpoints and restartpoints symmetrically. It also adjusts up
the documentation of the parameter to mention restartpoints.
Fujii Masao. Docs by me, as suggested by Itagaki Takahiro.
This allows the language-specific try/catch construct to catch and
handle exceptions arising from SPI calls, matching the behavior of
other PLs.
As an additional bonus you no longer get all the ugly "unrecognized
error in PLy_spi_execute_query" errors.
Jan Urbański, reviewed by Steve Singer
Heikki Linnakangas